Reversible lockup for flexible printing plate

ABSTRACT

A lockup for securing a flexible printing plate to a reversibly driven plate cylinder in which the leading end of the plate may be held in a reference position and in which the trailing end of the plate may be kept under constant tension regardless of the direction of cylinder rotation. A longitudinal groove is formed in the cylinder having undercut edges with a rockable shaft being mounted in the groove having a gripper extending longitudinally of the shaft and presenting gripping edges facing in opposite directions. An overcenter spring toggle detents the shaft in two positions in which the gripper lies on one side or the other of the center of the groove, manual throw-over means being provided for throwing the shaft between its detented positions so that, when a first end of the plate is hooked over one of the undercut edges and the second end of the plate is placed in the path of movement of the gripper, manual throw-over causes the gripper to swing into gripping engagement with the second end of the plate to keep the plate under constant tension on the cylinder. In the preferred embodiment the gripper is in the form of a leaf spring partially recessed in the shaft and symmetrical stops are provided for limiting rocking movement of the shaft in both directions.

United States Patent 1 Kostas et al.

[ 1 REVERSIBLE LOCKUP FOR FLEXIBLE PRINTING PLATE [75] Inventors: Evans Kostas, Hinsdale; Robert E.

Schwarl, Rockford, both of I11.

[73] Assignee: North American Rockwell Corporation, Pittsburgh, Pa.

[22] Filed: July 22, 1971 [21] App1.No.: 165,068

FOREIGN PATENTS OR APPLICATIONS 1,367,860 6/1964 France l0l/415.1

Primary Examiner-William B. Penn Assistant ExaminerWil1iam Pieprz Atzorney.lohn R. Bronaugh et a1.

(IMP/N6 1 Apr. 17, 1973 ABSTRACT A lockup for securing a flexible printing plate to a reversibly driven plate cylinder in which the leading end of the plate may be held in a reference position and in which the trailing end of the plate may be kept under constant tension regardless of the direction of cylinder rotation, A longitudinal groove is formed in the cylinder having undercut edges with a roekable shaft being mounted in the groove having a gripper extending longitudinally of the shaft and presenting gripping edges facing in opposite directions. An overcenter spring toggle detents the shaft in two positions in which the gripper lies on one side or the other of the center of the groove, manual throw-over means being provided for throwing the shaft between its detented positions so that, when a first end of the plate is hooked over one of the undercut edges and the second end of the plate is placed in the path of movement of the gripper, manual throw-over causes the gripper to swing into gripping engagement with the second end of the plate to keep the plate under constant tension on the cylinder In the preferred embodiment the gripper is in the form of a leaf spring partially recessed in the shaft and symmetrical stops are provided for limiting rocking movement of the shaft in both directions.

4 Claims, 9 Drawing Figures PATENTED 1 71975 3. 727. 551

sum 1 [1F 3 REVERSIBLE LOCKUP FOR FLEXIBLE PRINTING PLATE It is an object of the present invention to provide a reversible lockup for a flexible printing plate which accurately registers the leading end of the plate and which applies constant tension to the trailing end for either direction of cylinder rotation without any special modification or adjustment. It is a more specific object to provide, in a plate cylinder, a symmetrically arranged gripper having provision for detented throwover from a released position to a clamping position and in which the roles of the two positions are correspondingly reversed upon reversing the direction of rotation of the cylinder.

It is another object of the invention to provide a reversible lockup for flexible printing plates which is extremely simple, avoiding thosecomplications which have characterized prior efforts to secure symmetrical reversibility and which employs but a single gripper for gripping the trailing end of the plate for either forward or reverse rotation.

It is another object of the invention to provide a reversible lockup for flexible printing plates which is easy to operate, requiring only manual throw-over between alternate positions to effect clamping and unclamping regardless of direction of rotation and which may be used without exercise of particular care, or judgment on the part of the press operator.

It is still another object of the present invention to provide a lockup for a flexible printing plate which, in spite of its reversibility feature, imposes to a minimum upon the integrity of the outer surface of the cylinder, the width of the groove at the cylinder surface being reduced to that required for the hooking of the leading end plus insertion of the bent trailing end of the plate so that the cylinder presents a smooth, curved contour without substantial interruption resulting in minimum vibration when the cylinder is rotated in contact with a blanket cylinder and cooperating rollers.

It is a still further object of the present invention to provide a lockup for flexible printing plates which, in addition to its reversibility feature, is of universal application, being usable with flexible plates having a wide range of thickness as well as different physical characteristics. Thus the construction is usable both with heavy plates which tend to unroll and light plates which tend to tear. More specifically in this connection it is an object to provide a reversible lockup which is easily and quickly adjustable to vary the amount of tension.

which is applied to the plate and which includesmeans for distributing the tension uniformly over the bent, or

hooked, trailing end, notwithstanding the fact that the bend may be slightly cocked with respect to the axis of the plate and notwithstanding dimensional variations which may exist from point to point along the bend. It is a more detailed object to produce a lockup for a flexible printing plate which not only provides constant gross take-up at the trailing end upon elongation of the plate during running conditions but also equalization of applied stress from point to point along the trailing end.

It is yet another object of the present invention to provide, in a reversible lockup mechanism having capability of throw-over between alternate positions, a pair of stop surfaces for limiting the range of angular movement, which stop surfaces may be positioned remotely as in the preferred embodiment, or in near" position, as in an alternate embodiment, as may be desired to achieve two different modes of operation and tension adjustment.

Other objects and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which:

FIG. 1 is a fragmentary elevation of a printing cylinder employing the present invention following installation of a plate but prior to lockup.

FIG. 2 is a view similar to FIG. 1 but showing the mechanism thrown to its alternate or locked-up condition.

FIG. 3 is a fragmentary elevational view, in partial section, looking along the line 3-3 in FIG. 1.

FIGS. 4 and 4a are fragmentary sections showing the initial engagement of the trailing edge of the plate upon rotation of the shaft and the stressing of the leaf spring which occurs by reason of further rotation of the shaft.

FIGS. 5 and 6 correspond to FIGS. 1 and 2 but show the manner in which the plate is clamped for reverse rotation of the cylinder.

FIG. 7 is a diagram showing the variation in torque and force on the plate brought about by different adjustment of the toggle springs with the stops in the preferred remote position.

FIG. 8 is a diagram similar to FIG. 7 but showing the effect of employing stops in a near position.

While the invention has been described primarily in connection with a preferred embodiment, it will be understood that it is not intended to limit the invention to such embodiment but it is, on the contrary, the intention to cover the various alternate and equivalent forms of the invention included within the spirit and scope of the appended claims.

Turning now to the drawings, a plate cylinder indicated at 10 has ends 11, 12 and an outer surface 13. The cylinder is journaled for rotation on stubshafts I5, 16 fitted in suitable journals in the press frame. Formed in the surface of the cylinder is a longitudinally extending groove 20 having symmetrically arranged undercut edges 21, 22, the groove below the undercut edges being formed as a bore defining a longitudinally extending cavity 23 having a cylindrical wall 24. Mounted upon the cylinder is a flexible printing plate 30 having a first hooked end 31 which engages the undercut edge 21 and a second hooked end 32 which passes over the edge 22 of the cylinder and into the recess 23. The hooked end 31 may be considered as the leading end and the end 32 as the trailing end.

In accordance with the present invention a rockable shaft is mounted in the space 23 having a gripper mounted thereon which extends longitudinally of the shaft and which presents gripping edges facing in opposite directions, the shafthaving an overcenter spring toggle for detenting the shaft into positions in which the gripper lies on one side or the other of the groove so that when a first or leading end of the plate is hooked over one of the undercut edges and a second endof the plate is placed in the path of movement of the gripper, throw-over of the shaft to its alternate position causes the gripper to swing into gripping engagement with the second or trailing end of the plate thereby to keep the plate under constant tension as the press is operated. When the direction of the cylinder is reversed, the roles of the gripping edges are correspondingly reversed and throw-over is in the reverse direction, without requiring any modification or adjustment of the structure.

Thus referring to FIGS. 1 and 3, a shaft 40 of circular cross section is provided having ends 41, 42 which are fitted for attachment of the throw-over tool, being hexagonally shaped in the present instance. The shaft 40 is journaled for rocking action in a central bushing 43 and in corresponding end bushings 44, 45, which bushings are interrupted, or C-shaped, as indicated at 46 and held in registering phase position by respective set screws 47 for clearance of the longitudinal gripping member to be described. Since the bushings extend substantially more than 180 about the shaft, the shaft is held effectively captive, as if encircled by continuous bushings.

Extending longitudinally of the shaft is a gripper element 50 of T-shape presenting edges 51, 52 which extend, peripherally of the shaft, in opposite directions. The gripper is held in place in the shaft by means of a set of pins 55 which extend diametrically.

For the purpose of rocking the shaft 40 into alternative detented positions which lie on opposite sides of the groove 20, the shaft has, secured to its near" end as viewed in FIGS. 1 and 2, an arm 60 which is pinned by means of a pin 61 to a toggle link or rod 62, the end of which is both slidable and rockable in a guide or poppet 63, the latter having an integral stubshaft 64 (FIG. 3) parallel to the cylinder axis and which is fitted in a bore 65. Surrounding the rod 62 is a spring 66 which is seated against the guide 63 at one end and which, at its other end, bears against an adjusting nut 67 which is threaded on the rod and which has an accompanying lock nut 68. With the adjusting nut 67 screwed down against the spring, reaction pressure is exerted against the guide 63 so that the arm 60, and the shaft 40, are detented to one side or the other of an alined dead center position 69. The position 69 is so oriented that when the toggle is on dead center the gripper 50 is alined with the groove 20. The spring 66 acts to urge the gripper 50 either to the left of the groove, as shown in FIG. 1, or to the right of the groove as shown in FIGS. 2' and 5. A corresponding spring toggle structure is provided, in mirror image, at the opposite end of the shaft where similar elements are given the same reference numerals with addition of subscript a. Any suitable means may be employed for maintaining the arms 60, 60a in the same phase relation to the shaft; for example, the arms may be pinned to the shaft by through pins 59, 59a (FIG. 3). Providing separate spring toggles at each end of the shaft, adjusted for the same torque characteristic, minimizes the effect of torsional twisting of the shaft.

Although it is preferred to pin the arm 60 and rod 62 together and to make the end of the rod slidable in a guide 63 to accommodate endwise movement of the rod, it will be apparent to one skilled in the art that the rod, if desired, may simply be pivoted at 63, the relative endwise movement being taken up by slotting one or the other of the toggle members at the pin 61. In such event, however, the spring 66 would be interposed between the arm 60 and the adjusting nut.

In accordance with one of the aspects of the present invention the gripper 50 is made of resilient construction in the form of a single or composite leaf spring mounted in a recessed position in a longitudinal groove formed in the shaft 40. Referring, for example, to FIG. 4, the groove, indicated at 70, is preferably of V-crosssection extending deeply into the shaft, almost to the center, and the gripper 50 is of composite construction being formed of two leaves of spring steel 71, 72, each of which is of inverted L-shape arranged back to back and securely mounted in the root 73 of the groove. Making the gripper of resilient construction tends to equalize the forces applied from point to point along the hooked end 52 of the plate. The resilience overcomes the concentration of force which would tend to occur along the end of the plate if the bend or hook has been inaccurately formed and is not precisely perpendicular to the longitudinal axis of the plate. Moreover, any other dimensional variations which may occur from point to point along the end of the plate are accommodated by variations in the amount of strain in the gripper to avoid concentration of stress at the engaged high spots.

In accordance with one of the further aspects of the present invention stop surfaces are provided for limiting the range of swing of the arms60, 60a in each direction beyond the dead center position 69. In the present instance the stop function is taken care of by a single stop member for the arm 60 which cooperates with stop surfaces 81, 82 on the arm. Conveniently, the stop 80 may be in the form of a cap screw which is screwed into a threaded hole provided in the end wall 11 of the cylinder. It will be understood that at the opposite end of the cylinder a similar stop member (not shown) is provided cooperating with similar stop surfaces in identical phased relation. In the preferred form of the invention the stop and stop surfaces are so positioned that bottoming on the stop does not occur when the gripper is in tensioning engagement with the trailing end of the plate so that the force of the toggle spring 66 is constantly effective to produce tensioned take-up of the plate as elongation occurs particularly during the initial portion of a press run. That is, the stop elements are, in the preferred embodiment located remotely so as not to be engaged during normal running of the press while, nonetheless, permitting bottoming to occur when the gripper is thrown to its inactive or backed-off position. The stop surfaces therefore tend to severely limit the overtravel of the gripper in the backed-off direction which would occur if the toggle spring 66 were allowed to expand fully into its non-stressed state.

The effect of the stop surfaces will be made clear upon comparing FIGS. 1 and 2. It will be assumed in these figures that the plate cylinder is to operate in the counterclockwise direction. The plate 30 is assembled on the cylinder so that the leading end 31 thereof is hooked over the undercut edge 21 while the trailing end 32 of the plate is inserted into the groove into the relief space 23 (see FIG. 4) which is in the path of clockwise movement of the gripper 50. During the time that the plate is being assembled on the cylinder (FIG. 1) the shaft 40 is in its backed-off condition, with the link 62 and its spring 66 being above the center line 69, which forces the stop surface 81 into bottoming engagement with the stop member 80. A tool having a hexagonal socket, slipped on one of the ends of the shaft 40, is rocked in a clockwise direction thereby moving the gripping edge 52 into a pocketed position with respect to the hooked end 32 of the plate. FIG. 4 shows the condition of initial engagement between the gripper and the end of the plate. However, as the spring 66, having passed inwardly and downwardly beyond the dead center position 69, expands, the shaft 40 is rocked an additional amount clockwise which is accompanied by bending of the leaf spring elements 71, 72 which together form the gripper. An equilibrium condition is reached in whichthe torque exerted by the springs 66, 66a tending to rotate the shaft 40 in a clockwise direction is equaled by the reactive torque exerted by the stressed gripper 50, following which no further movement of the shaft may occur except that which might be permittedby the elongation of the plate.

As shown in FIG. 2, the stop surface 82 on the arm is so positioned with respect to the stop 80 that when using a plate of a given nominal length the end of the plate obstructs further rocking movement of the gripper shaft prior to occurrence of bottoming at the stop surface. The stop, in such circumstances, may be referred to as a remote" stop. That is to say, the stop is sufficiently remote from its cooperating stop surfaces so that it lies beyond the stop surfaces when the lockup is in its. working position. As a result the force developed in the toggle spring 66 is fully effective to provide follow-up movement of the arm 60, and the associated gripper, as the plate increases slightly in length, either due to the ironing effect upon the cylinder which occurs during the first few revolutions of the plate cylinder and cooperating blanket cylinder (not shown) or the elongation which may occur incident to a long press run. The remote stop is to be distinguished from what might be termed a near stop arrangement in -which bottoming occurs when the gripper is in its stressed working position. The present invention in its broader aspects contemplates both possibilities.

One of the advantages of employing a remote stop arrangement is that the force which is exerted upon the trailing end of the plate may be easily and quickly varied by rotation of the adjusting nuts 67 which vary the degree of pre-stress of the toggle springs 66, 66a. Thus by screwing down upon the nut 67 to increase the stress in the spring 66 the torque applied by the arm 60 upon the shaft may be increased, as may be required when using thick plates having high inherent strength. When using plates of thinner or less durable material the adjusting nut 67maybe appropriately backed-off to reduce the pre-stress in the toggle spring 66 so that a lesser degree of torque is applied by the arm 60 to the gripper shaft. It will be understood that whether the degree of pre-stress in the toggle spring arrangement is increased or decreased, the adjustment of the nuts 67, 67a at the respective ends of the shaft will be coordinated so that an equal torque is applied to the gripper shaft at each ofits ends.

The relationship of the torque exerted by the springs 66, 66a and thetorque exerted by the gripper spring on the plate using a remotely located stop is graphically illustrated in FIG. 7. Here the torque exerted by the springs 66, 66a upon the shaft 40 is indicated by the curve 90, the torque being plotted as the ordinate with the shaft movement from dead center as abscissa. It will be noted that the torque rises from zero at the dead center position, indicated at 91, along a rather steeply rising curve which reaches a maximum value with some drop-off, due to release of stress in the spring 66, as the shaft is additionally rotated.

For a plate of a certain nominal length, engagement of the edge 52 of the gripper will occur (FIG. 4) at a predetermined point 92 in the angular rocking movement of the shaft. As the shaft rotates further, and the toggle spring is permitted to expand, the torque developed in the gripper spring assembly, and which is applied to the plate, rises along a generally linear curve 93 until the point of intersection A is reached at which the torque exerted by the spring 66 and the reactive torque exerted against the gripper spring 50 are equal, at which time the mechanism is in equilibrium and no further movement occurs. The "remote position of the stop is illustrated by the vertical dashed line 95 which, as will be noted, lies beyond the point of intersection A by an amount 96 which corresponds to the spacing, between the stop surface 82 and the stop member shown in FIG. 2. The torque in the gripper, which corresponds to the force on the plate in the condition of equilibrium, is indicated in the diagram at D.

Where it is desired to increase the force upon the plate as, for example, when using; plates of thicker or more durable material, the degree of pre-stress in the spring 66 may be increased by screwing down upon the nut 67 resulting in a higher torque curve which has an intersection B with respect to the gripper spring characteristic 93. This corresponds to a greater equilibrium torque E and a correspondingly higher force applied to the end of the plate. Conversely, backing off the adjusting nut with respect to the toggle spring 66 drops the curve so that it intersects curve 93 at point C which corresponds to a lesser torque and lesser applied force F. It will be seen, then, that with a:

remotely located stop it is a simple matter to vary the tensioning force which is applied to the plate.

The curve 93 is that of a plate of a certain nominal length. There will be normal small variations in plate length, giving a gripper spring characteristic 97 for plates which are slightly short and 98 for plates which are slightly long. It will be noted, however, that in spite of variations in plate length, accompanied by lateral shifting of the points A, B and C in one direction or the other, as shown, there are only minor changes in the torque values E, D and F and therefore only minor changes in the forces applied to the plate for the three illustrated degrees of stress in the toggle springs.

The portion of FIG. 7 which has been discussed is applicable to rotation of the plate cylinder in the counterclockwise direction in which the toggle lies outside of the line of dead center 69 when the gripper is backedoff (FIG. 1) and inside of the line 69 when the gripper is stressed with the plate locked up.

When the direction of rotation of the plate cylinder is to be reversed, as illustrated in FIGS. 5 and 6 the plate 30, instead of being hooked over the undercut edge 21, is reversed and hooked, instead, over the undercut edge 22. The trailing end 32 of the plate is tucked into the groove so that it lies in the path of counterclockwise movement of the gripper. Comparing FIG. 5 to FIG. 1, while both of these figures show the gripper backed off, in one case the toggle lies outside of the center line whereas in the other it lies inside of the center line. It will also be noted that the toggle lies in opposite positions in FIGS. 2 and 6 which illustrate the clamped condition of the plate for the respective directions of cylinder rotation. In short, it will be noted that while there are only two detented positions of the toggle and of the shaft, their roles are reversed in the respective directions of cylinder rotation automatically and without requiring any mechanical modification or adjustment. Moreover, in the two conditions, the direction of movement of the shaft from dead center and the direction of the torque and the force applied to the plate are also both reversed. For clockwise cylinder rotation the torque characteristics 90', 93' apply, each of the numerals and letters having its primed counterpart, thereby illustrating the symmetry of structure and operation in the two modes of cylinder rotation.-

While we prefer to position the stop surfaces and stop member so that bottoming against the limit stop does not occur under running conditions, the invention is not limited thereto and the structure is susceptible to operation in a bottoming mode in which the bodily stressing of the gripper spring is relied upon to secure tensioned follow up as well as equalization of stress along the end of the plate. Thus, referring to FIG. 8, the torque curve of the toggle springs is indicated at 100 and the torque characteristic of the gripper spring is indicated atl03. This results in a tendency to achieve equilibrium at an intersection point H corresponding to a torque and force upon the plate indicated at K. Where, however, the stop 80 and the engaged stop surface 81 are located in a near" position, indicated at 105 in FIG. 8, the shaft cannot rotate beyond the stop and thus the point of equilibrium H is not achieved. Instead, the degree of flexure of the gripper spring is limited to an amount G which corresponds to a torque in the gripper spring, and a force applied to the plate, which is equal to the amount J.

If it is desired to limit the force against the plate to a still lower value, the stop 80 and stop surface 81 may be tailored to obstruct the shaft at the point 106, thereby limiting flexure of the gripper spring to that indicated at 1, corresponding to a reduced torque, and force against the plate, indicated at L in the figure. Effective movement of the location of the stop while preserving symmetry may be readily achieved by employing a cap screw 80 having a larger head provided, of course, that the arm 60 is accurately relieved to provide clearance for the larger head.

Just as in the case of using remote stop surfaces (FIG. 7) the spring characteristics 100, 103 have counterclockwise counterparts 100', 103 in the lower lefthand quadrant, all of the other corresponding letters and numerals being similarly primed.

While the invention has been described in connection with a gripper, or gripper spring, which extends continuously over the length of the shaft, it will be understood the gripper need not be continuous but may consist of a number of separate sections arranged end to end. Also while the gripper spring has been illustrated as two spring laminations arranged back to back, it will be apparent that the spring may, if desired, be formed of a single thickness of metal but presenting gripping edges extending in opposite peripheral directions. Or, alternatively, where separate gripper elements are used it will be apparent that they need not be closely engaged back to back but may be slightly separated without departing from the present invention.

It is one of the primary features of the construction that the lockup is symmetrical bi-directional, but it will be apparent to one skilled in the art that the construction may be used with advantage even in plate cylinders which rotate in a single direction.

Indeed, while the invention has been described in connection with a plate cylinder and a flexible printing plate, the term plate," as used herein, is intended to be a general term broad enough to cover any flexible sheet of durable material intended to be tightly fitted to the surface of a cylinder and hooked or bent for attachment at the leading and trailing ends.

Since the leading end of the plate is hooked over an undercut edge which is accurately formed and fixed in position, accurate and reproducible registry is assured; The actual plate clamping operation, involving only the swinging of a tool over dead center in one direction or the other, may be easily and quickly accomplished without exercise of any particular care or attention on the part of the operator. Adjustment of the springs 66, 66a when required to change the clamping force is a simple matter of adjusting the springs to the same effective lengths. While the gripper leaf spring is intended primarily for the purpose of distributing tensioning force along the leading end of the plate in spite of minor inaccuracies in the forming of the bend, it will be appreciated by one skilled in the art that, when using remote" stops, the springs 50, 66 are effectively in series with one another so as to increase the amount of available take up beyond that which would be provided by one of the springs used alone.

The invention has been described in connection with the simplest situation in which a single plate extends all the way around the cylinder, requiring only a single groove, and even that being of minimum width, so as to minimize impact and noise as the cylinder rotates against other cooperating cylinders and rollers. However, it will be apparent that the invention is fully applicable to use of two plates around" which requires that the lockup mechanism be duplicated at the position and in which case the leading end of one plate would occupy the same groove as the trailing end of the other. Consequently, the term the plate having leading and trailing ends, as used herein, shall be understood to refer to plate means in which the leading and trailing ends may be on two separate but adjacent plates.

The lockup described above has been found to be highly economical, consisting ofa minimum number of parts, with these parts being simply formed, to minimize the cost of manufacture and installation and providing a long life, substantially maintenance free.

What we claim is:

1. In a lockup for holding a flexible printing plate having hooked ends, the combination compromising a reversible plate cylinder defining a longitudinal groove having undercut edges, a rockable shaft in the groove extending along the length thereof, a gripper mounted on shaft extending longitudinally and presenting gripping edges facing in opposite directions, means including an overcenter toggle spring for detenting the shaft in two positions in which the the gripper lies on one side or the other of the center of the groove, and throw-over means for manually throwing the shaft between its detented positions so that when a first end of the plate is hooked over one of the undercut edges and the second end of the plate is placed in the path of movement of the gripper, operation of the throw-over means causes the gripper to swing into gripping engagement with the second end of the plate, the gripper being in the form of a leaf spring of T-eross-section having a central leg secured along its inner edge to the shaft and having arms extending in opposite directions.

2. The combination as claimed in claim 1 in which the leaf spring consists of two portions of inverted L- shape arranged back to back.

3. In a lockup for holding a flexible printing plate having hooked ends, the combination comprising a reversible plate cylinder defining a longitudinal groove having undercut edges and a cylindrical inner wall, a rockable shaft in the groove extending along the length thereof. longitudinally spaced bushings interposed between the shaft and the wall of the groove to provide bearing surfaces for the shaft, a gripper mounted on the shaft and extending longitudinally of the shaft presenting gripping edges facing in opposite directions, means including an overcenter toggle spring for detenting the shaft in two positions in which the gripper lies on one side or the other of the center of the groove, and throw over means for manually throwing; the shaft between its detented positions, the bushings being relieved adjacent the undercut edges and in the path of movement of the gripper to ,provide clearance for the gripper as the gripper moves into its respective detented posi tions.

4. The combination as claimed in claim 3 in which the gap between the undercut edges and the radial space between the shaft and the inner wall are of substantially minimum dimension consistent with the accommodation of the hooks on the ends of the plate. 

1. In a lockup for holding a flexible printing plate having hooked ends, the combination compromising a reversible plate cylinder defining a longitudinal groove having undercut edges, a rockable shaft in the groove extending along the length thereof, a gripper mounted on shaft extending longitudinally and presenting gripping edges facing in opposite directions, means including an overcenter toggle spring for detenting the shaft in two positions in which the the gripper lies on one side or the other of the center of the groove, and throw-over means for manually throwing the shaft between its detented positions so that when a first end of the plate is hooked over one of the undercut edges and the second end of the plate is placed in the path of movement of the gripper, operation of the throw-over means causes the gripper to swing into gripping engagement with the second end of the plate, the gripper being in the form of a leaf spring of T-cross-section having a central leg secured along its inner edge to the shaft and having arms extending in opposite directions.
 2. The combination as claimed in claim 1 in which the leaf spring consists of two portions of inverted L-shape arranged back to back.
 3. In a lockup for holding a flexible printing plate having hooked ends, the combination comprising a reversible plate cylinder defining a longitudinal groove having undercut edges and a cylindrical inner wall, a rockable shaft in the groove extending along the length thereof, longitudinally spaced bushings interposed between the shaft and the wall of the groove to provide bearing surfaces for the shaft, a gripper mounted on the shaft and extending longitudinally of the shaft presenting gripping edges facing in opposite directions, means including an overcenter toggle spring for detenting the shaft in two positions in which the gripper lies on one side or the other of the center of the groove, and throw-over means for manually throwing the shaft between its detented positions, the bushings being relieved adjacent the undercut edges and in the path of movement of the gripper to provide clearance for the gripper as the gripper moves into its respective detented positions.
 4. The combination as claimed in claim 3 in which the gap between the undercut edges and the radial space between the shaft and the inner wall are of substantially minimum dimension consistent with the accommodation of the hooks on the ends of the plate. 